Shield Defense v1.4

Big thanks for the front page entry!

Use your shield to protect your base from attacking tanks and repel their own projectiles back at them! Earn money by killing tanks and buy upgrades between levels to stay ahead of the game.

*Update - 07/02/07* BIG update with lots of new stuff:
- Difficulty levels
- Several new bonus upgrades, including an Antimatter Shield that takes care of that "multiple incoming bogeys" problem
- Music (thanks to MaestroRage!)
- Running out of time no longer ends the level, but the tanks will start firing much faster
- At Captain and General levels, energy type tanks take less damage from their own projectiles
- Your energy cells now recharge between levels
- Fixed stuck tanks bug and heatseeker save bug (sorry guys)
- A bunch of other tweaks

*Update - 06/28/07* Added a save game feature, due to popular demand. Thanks for all the votes and great reviews, guys! I really appreciate the praise, criticism, and suggestions.